The present invention relates generally to agricultural machinery having crop conveyors, and more particularly to a metal detecting device for a crop conveyor. In a preferred embodiment, several closely spaced permanent magnets are fixed within a hollow feed-roller in parallel with the shaft thereof.
A metal detecting device of this type is known from DE 43 01 611 A1. A plurality of permanent magnets are arranged in a row, transversely of the crop feed path, within a hollow feed-roller. One magnetic pole of each magnet is directed toward the feed-roller and is located close to the inner surface thereof. The permanent magnets are surrounded by respective sensing coils and the other poles of the magnets are accommodated in a U-shaped yoke whose two ends terminate close to the inner face of the feed-roller. By this arrangement, the basic sensor field extends through the feed-roller into the adjacent feed-slot to a depth corresponding approximately to half the width of the yoke. The signals from the sensors are added to form the wanted signals and, to compensate for interference signals which arise, in particular, from the movement of the feed ribs of the feed-roller in each sensing field, they are subtracted from one another and then used as reference values. A disadvantage of this arrangement is the relatively small range covered by the sensing fields since there are limits to the amount by which the yoke can be enlarged. This is because the rotating members, which interfere heavily with the sensors, are located at the entry point of the feed-slot. Another disadvantage is that the detection process at the output of the feed-slot occurs too late to allow the conveyor to be stopped before a detected piece of metal reaches the subsequent chopper.